Cryopumps create exceptionally-low-pressure vacuum conditions by condensing or adsorbing gas molecules onto cryopanels cooled by cryogenic refrigerators. Commonly, a cryopump used in this context is cooled by a refrigerator performing a Gifford-McMahon cooling cycle. These refrigerators generally include one or two stages, depending upon which gases are sought to be removed from the controlled atmosphere. Two-stage cryopumps are used when removal of low-condensing-temperature gases, such as nitrogen, argon and hydrogen is desired. The second stage is typically operated at approximately 15 to 20 K to condense these gases upon a cryopanel thermally coupled to the second stage of the refrigerator.
In contrast, a single-stage cryopump, sometimes termed a waterpump, is typically operated at warmer temperatures than the second stage of a two-stage cryopump, typically at about 107 K. Operating at this temperature, a single-stage cryopump will nearly eliminate the presence of water vapor.
In the refrigerator of a typical cryopump, the flow of compressed-gas refrigerant is cyclic. A compressor supplies compressed gas to the refrigerator through a supply line leading to an inlet valve. An exhaust valve leading to an exhaust line returns the refrigerant from the refrigerator to the low-pressure inlet of the compressor. Both valves are located at the first end of a cylinder within the refrigerator. At the opposite, second end of the cylinder, a thermal load, including a cryopanel, is thermally coupled to the cylinder.
With a displacer, including a regenerative heat exchange matrix (regenerator), at a second end of the cylinder, and with the exhaust valve closed and the inlet valve open, the cylinder fills with compressed gas. With the inlet valve still open, the displacer moves to the first end to force compressed gas through the regenerator, the gas being cooled as it passes through the regenerator. The inlet valve is then closed and the exhaust valve is opened, and the gas expands into the low-pressure exhaust line and cools further. The resulting temperature gradient across the cylinder wall at the second end causes heat to flow from the thermal load to the gas within the cylinder. With the exhaust valve opened and the inlet valve closed, the displacer is then moved to the second end, displacing gas back through the regenerator which returns heat to the cold gas, thus cooling the regenerator and completing the cycle.
As layers of condensed gases accumulate upon the cryopanel, the effectiveness of the cryopump is gradually compromised, and the volume of available pumping space may be depleted. To remedy this loss, both single-stage and two-stage cryopumps are routinely subjected to regeneration procedures. During a typical regeneration procedure, the cryopanel, which is coated with a layer of condensed gases, is warmed well above its operating temperature to sublimate or to liquefy and evaporate the gases condensed upon it. The liberated gases are typically removed from the surrounding vacuum chamber by a rough pump, and the cryopanel is returned to its cold, operating temperature. The regeneration procedure thereby cleans the surface of the cryopanel of accumulated condensates.
A selective sublimation regeneration procedure is performed where a toxic or acid forming gas such as chlorine is condensed on a cryopanel. The selective sublimation regeneration procedure releases and removes the toxic or acid forming gas to limit the accumulation of the toxic gas upon the cryopanel and the interaction between the toxic gas and water vapor. The toxic or acid-forming gas is selectively removed from a cryopanel by warming the cryopanel to a temperature within a selective defrost range. At temperatures within this selective defrost range, the toxic or acid-forming gas selectively releases from the cryopanel as a vapor while water remains substantially condensed upon the cryopanel. The temperature of the cryopanel is maintained within this range until the toxic or acid-forming gas is substantially released from the cryopanel and removed from the surrounding chamber. The range of selective defrost temperatures at which the cryopanel is maintained is below the triple point of the electively removed gas so that it is released as a gas and not liquid.
During a typical shutdown procedure of a cryopump, the refrigerator is turned off and heaters are turned on to warm the cryopump to room temperature. Then, the cryopump is turned off. In semiconductor etching processes, where toxic or acid forming gases are routinely used, the shutdown procedure melts the condensed gases on the cryopanel and forms a liquid that may be hazardous or corrosive. A selective regeneration procedure may be performed before shutdown to sublimate toxic or acid forming gases prior to turning off the cryopump to avoid the melting of condensed gases and subsequent formation of a hazardous liquid. After the regeneration cycle is completed but before the cryopanel is cooled back down to operating temperature, the cryopump is turned off.
In the event of a power failure, a cryopump may be set to follow a power failure recovery procedure after power recovers, such as disclosed in U.S. Pat. No. 5,157,928. The system first determines whether the cryopump was on, off or in regeneration when the power went out. If the cryopump was off, then the cryopump remains off. If the pump was on, then the system checks to determine whether the sensed temperature is sufficiently low to permit a successful restart of the cryopump and, if so, to start the refrigerator motor. If the temperature is above a set point temperature, the system initiates a regeneration cycle. If the cryopump was in regeneration, the system determines whether the cryopump was in the process of cooling down. If the cryopump was not cooling down, regeneration is restarted. If the cryopump was cooling down, the system determines whether the sensed pressure is sufficiently low to continue the original regeneration cycle and, if so, cool down is continued. If the sensed pressure is not sufficiently low, regeneration is restarted.
In a system having a sublimation regeneration mode, the power failure recovery procedure may be different. In particular, different steps may be followed where the cryopump was in regeneration or cryopump on. Unlike the power failure recovery procedure of non-sublimation regeneration systems, if the cryopump was in regeneration, the system further determines whether the cryopump was in the process of sublimating. If the cryopump was sublimating, the system determines whether the sensed temperature is above or below a set point temperature. If the temperature is above the set point, the cryopump remains off. If the cryopump was not sublimating, the system then determines whether the cryopump was cooling down. The system also determines whether the cryopump was cooling down where the cryopump was sublimating but the sensed temperature is below the set point. If the cryopump was cooling down, the system continues the cool down process without checking pressure. If the cryopump was not cooling down, regeneration is restarted as before. As in the non-sublimation recovery, if the cryopump was on and the system determines that the sensed temperature is below a set point temperature, then the cryopump is turned on. However, if the sensed temperature is above the set point, the system determines whether the recovery procedure or cool down is to be followed as preset by the user. If the recovery procedure was selected, then the cryopump is turned on. If the cool down process was selected, then the cryopump remains off.
A power failure recovery, regeneration or shutdown may cause damage to the system and may present health risks if performed improperly. A toxic or acid forming gas, such as chlorine gas (Cl2), is routinely used in semiconductor etching processes which commonly incorporate the use of a cryopump. When toxic gas is present in a chamber of a process tool where the cryopump is operating, the toxic gas typically condenses upon the cryopanel along with condensed water.
In the event of a power failure, regeneration or shutdown, toxic or acid forming gases are liberated. The liberated gases routinely intermix and react with one another to form corrosive and hazardous liquid. For example, chlorine reacts with water to produce hydrochloric acid. Hydrochloric acid is highly corrosive, and, therefore, may damage the chamber, the work pieces within it, and the cryopump. Moreover, the production of hydrochloric acid creates disposal problems as well as a health hazard for individuals in contact with the process tool. If left to accumulate unabatedly, a dangerous amount of corrosive or hazardous liquid can be left in the chamber after the refrigerator has warmed, or in the event of a power failure.
Typical power failure recovery procedures fail to remove or limit the accumulation of gases and liquid remaining after a power failure. To sufficiently remove liquid, a sublimation regeneration process should be followed. Since the power recovery procedures typically perform a non-sublimation regeneration, gases and liquid remain in the cryopump after the power recovery is completed.
In a system having a sublimation regeneration mode, the power recovery procedure also may fail to sufficiently remove or limit the accumulation of gases and liquid remaining after the power failure. For example, a liquid formed during a power outage remains in the cryopump where the cryopump was in regeneration before the power failure but stays off because it is determined that the cryopump was sublimating and the sensed temperature is above the set point. Similarly, the recovery procedure fails to remove liquid where the cryopump was on before the power failure but stays off after power recovery because the sensed temperature is above the set point and the user selected the cool down. Further, both recovery procedures fail to provide a recovery process where the cryopump was in a shutdown process before the power failure.
The risks presented by corrosive and hazardous liquids can be minimized by limiting the accumulation of liquid and removing the liquid during a recovery from a power failure by always turning the refrigerator back on during recovery. In accordance with one aspect of the invention, when power recovers after a power failure, the operating state of the cryopump before the power failure and present conditions of the cryopump are determined. More specifically, depending on the operating and present conditions of the cryopump, a regeneration or startup process is initiated.
The regeneration process is initiated where there may be an accumulation of corrosive or hazardous liquid or toxic forming gases remaining in the cryopump. For example, if the cryopump was on before the power failure but the present temperature of the cryopump is above a set point temperature, then the regeneration process is initiated to remove any gases or liquid from the cryopump. The set point temperature may be between 110 and 260 K. The regeneration process is also initiated where the cryopump was in a regeneration cycle before the power failure but the present pressure of the cryopump is above a predetermined pressure level.
Regenerations are used to sublimate toxic or acid forming gases from the cryopump. The regeneration process includes refrigerating the cryopanel to a temperature within a defrost range and maintaining the temperature until the gases are removed from the cryopump. The range of defrost temperatures at which the cryopanel is maintained is below the triple point of the gases being removed. The defrost temperature may be set at about or less than 250 K. The gases are substantially removed and the cryopump is clean when the pressure of the cryopump drops to the predetermined pressure level. The set point pressure level may be about 100 microns. After the removal of condensed gases, the cryopanel may be cooled down to the operating, cryogenic temperature.
If the cryopump was in a shutdown procedure before the power failure, then a regeneration cycle may be initiated. After the regeneration cycle, the cryopump may be warmed to about 310 K. The cryopump is then turned off.
The startup process includes cooling down the cryopump to the operating temperature. The startup process is initiated where the cryopump was in a startup process before the power failure. The startup process is also initiated where the cryopump was on before the power failure and the present temperature of the cryopump is below the set point temperature. If the cryopump was in a regeneration process before the power failure and the present pressure of the cryopump is below the predetermined pressure level then the startup process is initiated when power recovers.
The cryopump may be cooled by a single stage refrigerator performing a Gifford-McMahon cooling cycle. The temperature at which the cryopanel is maintained during operation may be between 50 and 150 K.
A shutdown procedure may include a regeneration cycle to prevent the accumulation of corrosive or hazardous liquid in the cryopump.
An electronic module may be programmed to determine the operating state before a power failure and present conditions of a cryopump and initiate a regeneration or startup process depending on the operating state and present conditions of the cryopump.